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Selection for Resistance to Sclerotinia Crown and Stem Rot in Alfalfa

Dep. of Plant Pathology
Dep. of Horticulture and Crop Science, 2021 Coffey Rd., The Ohio State Univ., Columbus, OH 43210-1087

^* Corresponding author (pierson.4{at}osu.edu).

Efficient selection methods are needed for improving resistance in alfalfa (Medicago saliva L.) to Sclerotinia trifoliorum Erikss., the causal agent of Sclerotinia crown and stem rot (SCSR). The objective of this research was to determine if SCSR resistance in alfalfa could be improved by recurrent phenol) pic selection. Selections and evaluations were conducted at Columbus, OH, between August 1991 and April 1994 in three alfalfa populations, ‘Armor’, ABI 9023, and ABI 9030. Seedlings were inoculated with the pathogen in the growth chamber and field. Surviving plants were recovered and intermated for each cycle of selection. A standard seedling test was used to measure the progress of selection. Two cycle-3 populations and the base populations from which they were derived were also inoculated with five additional isolates of S. trifoliorum to examine the durability of resistance in the selected populations. Progeny of populations selected for two or three cycles in the growth chamber a expressed significant (P = 0.05) reduction in disease severity index (DSI). Generally, progeny of one cycle of field selection were not significantly different than the unselected base population. However, in Armor, one cycle of selection in the field (DSI 3.39) was comparable to two or three cycles in the growth chamber (DSI 3.41). Realized heritability estimates for resistance to SCSR ranged from 0.07 to 0.27. Isolates of S. trifoliorum differed in virulence, but there were no isolate by population interactions. The growth chamber selection method developed, using a single isolate of S. trifoliorum, was effective in increasing resistance in three alfalfa populations.

Received for publication June 21, 1996.